Synchronous motor



April 16,1935. H, E ARREN 1,998,297

SYNCHRONOUS MOTOR I Filed May 26, 1934 Inventor: d Henry E. War n, y W 8Hrs Attorney.

Patented 16, i935 SYNCHRONOUS Moron Henry E. Warren, Ashland, Mara,assignor to Warren Telechron Company, a corporation of Maine ApplicationMay 26, 1934, Serial No. 727,749

2 Claims. (01. ire-275) My invention relates to synchronous motors andits object is to provide an inexpensive form of low-speed synchronousmotor suitable for driving clocks. The motor to be described is of thenonself-starting variety requiring means other than its synchronousmotor features to bring it up to approximatelysynchronous speed. Such amotor is usually started by spinning the rotor by hand and the motor ofmyinvention may be started in this way.

A simple form of motor according to my invention may consist 01 twoaxially-displaced motor sections, each consisting of a magnetic toothedrotor part and a bipolar stator field part which usually has teeth inthe pole faces. The axiallydisplaced rotor parts are joined byan axialmagnet core which may be a permanent magnet and the axially-displacedstator parts are extensions from a common core energized by asingle-phase coil.

The spacing of the teeth in the rotor parts and in each main stator polepart are the same, but, due to a staggered relationship between theteeth in the stator and rotor parts to be hereinafter explained, thealternating fiux of the stator takes a path of minimum reluctancethrough the rotor at synchronous speed which is in an axial directionand always in the same axial direction. Due to this relationship, therotor becomes polarized in an axial direction and may, if desired, bepermanently magnetized in such direction.

The synchronous speed of such a motor can be made most anything desiredby varying the number of rotor teeth and by making corresponding changesin the stator. Other things being equal, the diameter of such a motor isabout half that of earlier synchronous motors for the same synchronousspeed.

The features of my invention which are believed to be novel andpatentable will be pointed out in the claims appended hereto. For abetter understanding of my invention, reference is made in the followingdescription to the accompanying drawing in which Fig. 1 represents anend view of a motor built in accordance with my invention; Fig. 2 is apartially sectioned side view of such a motor; Fig. 3 is a diagrammaticview of the motor arranged to illustrate the flux paths therethrough atsynchronous speed; and Fig. 4 is a perspective view of a simplehigh-speed embodiment of the invention where the stator teeth arereduced to one per pole.

Referring particularly to Figs. 1 and 2, it will be noted that the rotorof the motor comprises a pair of axially-displaced discs ll and-l2 whichhave their central portions magnetically connected by an axial core partl3. These rotor parts may be made from hardened steel and the part l3may be and usually is a permanent mag- 5 net polarized in an axialdirection although this is not essential. It may be stated, however,that the rotor tends to become permanently magnetized in an axialdirection, during synchronous operation in any event, because the fluxthere- 10 through is always in the same direction. The rotor discs havethe same number of evenlyspaced teeth in their periphery. The number ofsuch teeth will depend upon the synchronous speed desired and thedimensions and propor- 15 tioning of such teeth should be such as toproduce satisfactory synchronous torque in this type of motor. The rotoris mounted on.a shaft 9 and 7 supported in suitable bearings indicatedat 8.

The stator of the motor consists of a pair of 20 axially-spaced bipolarfield elements l4 and I5 extending from a common core part I! andenergized from a common single-phase coil IS. The bipolar field elementsare thus magnetically connected in parallel. The statorthushasfour mainpole pieces l8, I9, 20, and 2| (see Fig. 3). Pole pieces l8 and 20 arepositive when pole pieces l9 and 2| are negative and vice versa. Thepolar portions of the stator 01 each field section are preferably joinedby a thin portion 22 to strengthen the structure mechanically withoutshunting any material amount of flux from the pole pieces.

The pair of bipolar axially-spaced field members l4 and I5 substantiallyembrace and cooperate with the pair of axially-spaced rotor discs H andI2 and each of the stator pole pieces have teeth out therein adjacentthe rotor discs. The teeth in each stator pole piece are spaced the sameas the teeth in the rotor but, as best shown in Fig. 3, the teeth in thedifferent pole pieces are staggered in such a way that (considering themotor field to be energized) the teeth in one rotor disc I I areopposite the teeth in the adjacent negative stator pole piece I8 andbetween the teeth in the adjacent positive stator pole piece l9 at thesame time that the teeth in the other rotor disc l2 are opposite theteeth in the adjacent positive stator pole piece 2t and be-- tween theteeth in the adjacent negative stator pole piece 20 as represented inFig. 3. At this instant then, the path or minimum reluctance torthestator'fiux through the rotor is from positive stator pole piece 2| intorotor disc l2, axially through the rotor core It to the other rotor discH, and out into negative stator pole-piece l8. This flux path isindicated by the full-line arrows in Fig. 3. If new the rotor is runningat a speed where its teeth move half their spacing in a half cycle, theywill arrive at another minimum reluctance position in time to interceptthe next flux pulsation which is in the opposite direction in the statorpoles but which passes through the rotor in the same axial direction asbefore. This is because stator pole pieces l9 and 2! will then benegative, stator pole pieces l8 and 28 willjbe positive, and the rotorteeth are displaced one-half tooth pitch from the positions representedin Fig. 3 so that the path of the flux through the rotor will be frompositive stator pole piece 20 into rotor disc !2', axially through rotorcore i3 to rotor disc H, and out to negative stator pole piece i9 asindicated in the dotteddash line arrows in Fig. 3. The motor will thushave a definite synchronous speed where the rotor teeth move a halftooth pitch per half cycle or a full tooth pitch per cycle because atthis speed the reluctance between stator and rotor pulsates betweenmaximum and minimum values in synchronism with the flux pulsations. Atsuch speed, all the stator flux pulsations of one sign pass through orutilize stator pole pieces l8 and 2! while all the stator fluxpulsations of opposite sign utilize stator pole pieces l9 and 20. Allthe flux pulsations pass through the rotor in the same axial direction.

The reluctance between a rotor disc and its two adjacent stator polepieces varies between the two pole pieces alternately with the fluxalternations. For example, disc l2 receives a positive flux pulsationfrom pole piece 2| when the reluctance therebetween is a minimum andwhen the reluctance between this disc and stator pole 20 is a maximum.The next half cycle disc i2 receives a positive flux pulsation fromstator pole 20 when the reluctance therebetween is a minimum and whenthe reluctance between disc l2 and pole piece 2! is a maximum.

From the above, it will be seen that there are two importantdistinctions between this form of motor and the ordinarynonself-starting synchronous motor of the reluctance type such as isdescribed, for example, in United States Patent No. 1,924,853 toHammond, August 29, 1933.

My motor runs one-half the speed of the ordinary reluctance motor forthe same size of rotor disc and teeth because, in the ordinaryreluctance motor, the teeth of the rotor move a complete tooth pitchduring a half cycle instead of onehalf tooth pitch. Relatively speaking,in this new motor, I have the same number of rotor teeth as in theordinary reluctance motor for the same synchronous speed but here theseteeth are on two discs of half the diameter of the single disc of theordinary reluctance motor. The diametrical dimensions of the motor maythus be reduced by half to obtain the same synchronous speed as beforeby means of the present invention. As.

shown in Fig. 2, the axial dimensions of the motor can be made verysmall and the reduction in diameter is much more important than theslight increase in length in reducing the overall dimensions of themotor and arriving at a compact design.

The second important distinction of my motor over the usualnonself-starting reluctance motor is that the flux through the rotor isalways in the same direction and, hence, can be made of hardened steeland become more or less permanently magnetized by the flux pulsations atsynchronous speed, thereby increasing the synchronous pull-out torque,reducing the necessary stator excitation, and improving the power factorof the motor. In fact, the axial rotor core part !3 can be made apermanent magnet using a magnetic alloy of high retentivity that willretain its permanent magnetism although of very short dimensions in thedirection of its polarization. To the extent that the rotor is orbecomes permanently magnetized, the result is analogous to that obtainedwhere direct current excitation is employed in large synchronous motors.This polarization or remnant magnetism in the rotor is beneficialbecause the pure reluctance torque is supplemented by torque due to theattraction and repulsion effect of unlike and like magnetic poles.

If the rotor core is not initially permanently magnetized, the rotorwill, when intially synchronized, pull intostep as a reluctance motorwithout regard to the axial direction of the flux therethrough. However,after being permanently magnetized either initially or by previoussynchronous operation, it will always synchronize in accordance with thepolarity of the remnant magnetism in the rotor.

The motor as shown in the drawing is provided with twenty-one teeth ineach of its rotor discs. Such a rotor will, therefore, make a completerevolution in twenty-one cycles at the basic synchronous speed and on a60 cycle excitation will have a basic synchronous speed of 171%revolutions per minute. If the rotor has twenty teeth, its synchronousspeed will be 180 revolutions per minute on 60 cycles. With twenty-tworotor teeth, the synchronous speed will be 163% revolutions per minuteon 60 cycles. Thus another advantage of this motor is that we are nowable to obtain a variety of synchronous speeds not heretofore obtainablein a very economical and simple manner. Also, we are able to obtainsynchronous speeds with a polarized rotor corresponding to various oddnumbers of poles which, so far as I am aware, has never before beenaccomplished.

As the number of rotor teeth are reduced, the number of stator teethshould be reduced to the extent necessary to maintain a properlyproportioned motor. For example, with a rotor having only three salientpoles in each disc, the number oi teeth in each stator pole would bereduced to one resulting in a simple bipolar salient pole structure atthe two ends of the rotor. 'The synchronous speed would be 600revolutions per minute on 60 cycles. Such a construction is representedin outline in Fig. 4 and perhaps represents the simplest embodiment ofthe invention and also the highest speed for which the invention ispracticable. In all cases, the stator has substantially the simplicityof an ordinary single-phase bipolar structure.

Owing to the ability of obtaining odd synchronous speeds in a simplemanner, it may be desirable in some cases to manufacture this motor inlarger sizes than is v at present contemplated. In such instances, itinay be desirable to excite the rotor with a direct current winding.This may be done in a very simple manner as I have indicated in Fig. 3where 23 represents a stationary direct current exciting coilsurrounding the rotating axial core l3.

What 1 claim as new and desire to secure by Letters Patent of the UnitedStates, is: r

1. An alternating-current synchronous motor comprising stator and rotormembers, the stator member comprising two parallel plates, each platebeing substantially magnetically and symmetrically divided into twoparts and the magnetically divided parts of both plates being joined bya common magnetic core piece, a single-phase energizing winding on saidcore piece whereby opposite polarity fluxes may be simultaneouslyproduced in the magnetically divided portions of each plate, themagnetically divided portions of each plate approaching each other at apoint removed from the core so as to form with said core a pair ofbipolar field structures, the rotor for said motor comprising twoparallel magnetic discs mounted for rotation as a unit on a common axis,said discs being respectively in the planes oi. said parallel statorplates and interposed between the bipolar portions thereoi, the centralportions oi said discs being magnetically connectedwtogether, thebipolar portions of each stator plate having a plurality of evenlyspaced teeth facing the rotor discs and the rotor discs having aplurality oi. similarly spaced teeth facing the stator teeth, the teethin the different bipolar portions-oi the stator being suiiicientlystaggered with respect to each other that, when the singlephase statorcoil is energized, the alternating stator fluxes find paths of minimumreluctance through the rotor always in the same axial direction from onedisc to the other at a speed where Y the rotor turns the distance equalto the spacing between its teeth per flux cycle.

2. An alternating-current synchronous motor comprising stator and rotormembers, the stator member comprising two parallel plates. each platebeing substantially magnetically and symmetrically divided into twoparts and the magnetically divided parts 0! both plates being joined bya common magnetic core piece, a single-phase energizing winding on saidcore piece whereby opposite" polarity fluxes may be simultaneouslyproduced in the magnetically dividedportions of each plate, themagnetically divided portions of each plate approaching eachother at apoint removed from thecore so as to form with said core a pair ofbipolar field structures, the rotor for said motor comprising twoparallel magnetic discs mounted for rotation as a unit on a com- .monaxis, said disc being respectively in the planes 0! said parallel statorplates and interposed between the bipolar portions thereoi, the centralportions or said discs being Joined by an axially disposed permanentmagnet, the bipolar portions oi. each stator plate having a plurality ofevenly spaced teeth racing the rotor discs and the rotor discs having aplurality of similarly spaced teeth facing the stator teeth, the teethin the diiierent bipolar portions of the stator being sumcientlystaggered with respect to each other that, when the single-phase statorcoilis energized, the alternating stator fluxes find paths of minimumreluctance through the rotor always in the same axial direction from onedisc to the otherat a speed where'the rotor turns the distance equal tothe spacing between its teeth per flux cycle.

HENRY E. WARREN.

